A differential gravimeter measures a gravity variation between two locations. In addition, multiple differential gravimeters can be used to develop a three-dimensional map of gravity. Such 3-D mapping has been proposed in order to monitor fluid flow in-situ in subterranean reservoirs, such as an oil field. In order to be used in an in-situ underground application, however, the gravity sensors of a gravimeter must be directly insertable into boreholes of the oil field. As a result, the gravity sensors must be small and robust.
Since the differences in gravity across an oil field are typically very slight, the gravimeter must be extremely sensitive; sensitivity below 1 micro-Galileo is often necessary. Such extreme sensitivity, however, requires very high immunity to noise sources. Error can be introduced into the output signal of a gravimeter from noise sources such as electromagnetic interference, horizontal components in the acceleration of a free-falling mass, mechanical misalignment of sub-components, mechanical shock, and Coriolis forces that arise due to the rotation of the Earth.
Gravimeters have been developed that are based on the principle of balancing the weight of a fixed mass with forces from a normal or superconducting spring. Gravimeters such as these, however, have gravity sensors that are typically too large to be inserted into a borehole of an oil well. They are also difficult to setup and calibrate. In addition, they are sensitive to environmental influences such as temperature and vibration.
More recently small gravimeters have been developed that include gravity sensors specifically designed for direct insertion into a borehole. These small gravimeters utilize piezoelectric launchers to vertically launch a pair of test masses upward so that they can subsequently free-fall downward. An interferometer arrangement is used to monitor the acceleration of their falling masses after each reaches its apex. In addition to some of the drawbacks of other prior-art gravimeters, however, noise due to shock and vibration caused by their piezoelectric launchers limits the sensitivity of these gravimeters.
There exists a need, therefore, for a gravimeter that avoids or mitigates some or all of the problems associated with prior-art gravimeters.